Non-volatile memory devices that retain stored data in the absence of power are pervasively used in many consumer electronic products including cell phones, tablets, personal computers, personal digital assistants, and the like. Unfortunately, many non-volatile memory devices have limitations that make them unsuitable for use as primary storage for these products including higher cost and lower performance when compared to volatile memory devices such as dynamic random access memory (DRAM). Examples of older technology non-volatile memory devices include read-only memory (ROM) and flash memory. Examples of newer technology non-volatile memory devices include resistive random access memory (RRAM), phase change memory (PCM), spin-transfer torque magneto resistive random access memory (STT-MRAM), ferroelectric random access memory (FRAM), and many others. RRAM operates on the basis that a typically insulating dielectric may be made to conduct through formation of a conduction path or filament upon application of a sufficiently high voltage. Formation of the conduction path may occur through different mechanisms, including defects and metal migration. Once the conduction path or filament forms, the filament may be reset (broken, resulting in high resistance) or set (reformed, resulting in lower resistance) by an appropriately applied voltage. Recent data suggests that the conduction path may include many current paths, rather than a single path through a single filament.
RRAM memory devices including conductive bridge RAM (CBRAM) and transition metal oxide RRAM are a focal point for current development. In CBRAM devices, metal filaments between two electrodes form the conduction path, where one of the electrodes participates in the reaction. In transition metal oxide RRAM, oxygen vacancy filaments in a transition metal such as hafnium oxide or tantalum oxide form the conduction path.
RRAM memory devices are often in use to store data or executable code in embedded applications having logic circuitry including core transistors. The voltage required to write data in RRAM memory devices may be higher than that required to operate the core transistors. A challenge to the use of RRAM memory devices in embedded applications, therefore, is to find a select transistor configured to select a cell in the RRAM memory device whose operational parameters are consistent with that of core transistors.
Input/output (I/O) transistors common in logic circuitry may be used as select transistors since I/O transistors may handle the high voltage requirements of RRAM memory devices. I/O transistors are disadvantageous as select transistors, however, because they have a large footprint that increases the cost of manufacture. A need remains, therefore, for an improved RRAM memory device including area efficient select transistors capable of handling higher voltages for use in embedded applications.